1. Field of the Invention
The present invention relates to a toner useful for developing electrostatic latent images formed by an image forming method such as electrophotography, electrostatic recording and the like.
2. Discussion of the Background
In order to fix a dry toner image formed on a receiving material by electrophotography and the like, a pressure-heating method, in which a heated fixing roller contacts the toner image upon application of pressure, is typically used. This method has an advantage in that high speed fixing is possible because the method has good heat efficiency. However, the method has a drawback in that a so-called "offset problem" tends to occur in which a part or all of toner images adheres to the surface of a fixing roller and then the adhered toner images are re-transferred onto another images because the surface of the fixing roller contacts the melted toner images while a pressure is applied to the toner images.
In attempting to solve this offset problem, a fixing method is typically used which uses a fixing roller, the surface of which is formed of a material having a good releasing ability such as silicone rubbers or fluorine-containing resins and on which a releasing oil such as silicone oils is coated. This fixing method is very effective for preventing the offset problem. However, the method has a drawback in that a device for supplying the releasing oil is needed in a fixing unit, and therefore the fixing unit becomes large in size, resulting in high manufacturing cost.
Accordingly, in recent years a method has been used in which a releasing agent is included in a toner without coating a releasing oil on a fixing roller. However, when a releasing agent is included in a toner, problems which occur are that the ability of the toner to be transferred onto receiving materials (hereinafter this ability is referred to as transferability) deteriorates and that the charging ability of a friction-charging member such as a carrier deteriorates because the friction-charging member is contaminated by the releasing agent which bleeds out of the toner particles.
In addition, since the particle diameter of toner particles becomes smaller and smaller in recent years to obtain images having good resolution, it is not easy to satisfactorily transfer such fine toners onto receiving materials.
In attempting to improve the transferability, a method is disclosed in which an ultra fine particulate material which has a particle size on the order of submicrons and which is hydrophobized is externally-added to a toner (i.e., is mixed with toner particles). By this method, the area of the surface of the toner to be contacted with a fixing roller decreases because the ultra fine particulate material adheres on the surface of the particles, and therefore the adhesive power of the toner to the fixing roller decreases. However, when an ultra fine particulate material is externally added to a toner, i.e., a particulate material is present on the top surface of a toner, the following problems tend to occur:
(1) the charging quantity of the toner seriously changes depending on environmental conditions such as temperature and humidity; PA1 (2) a photoconductor to be contacted with the toner, easily deteriorates because the photoconductor is abraded or a film is formed on the surface of the photoconductor, each of which is caused by the toner; PA1 (3) the fixability of the toner deteriorates; and PA1 (4) the transferability of the toner deteriorates when used for a long time because the particulate material becomes to be buried in the toner particles over the course of time. PA1 (1) since the deference between the melt viscosities of the binder resin and the releasing agent decreases because the melt viscosity of the releasing agent increases due to the addition of the particulate material therein, the releasing agent is uniformly dispersed in the toner particles while forming miniaturized domains when these materials are kneaded to prepare a toner; and PA1 (2) the releasing agent is not easily deformed or crushed because the releasing agent is reinforced by the particulate material. PA1 (1) two grams of a releasing agent is melted by being heated at a temperature higher than its melting point: PA1 (2) a particulate material is added to the melted releasing agent little by little while they are kneaded with a spatula so that the mixture forms an aggregation: and PA1 (3) the particulate material is continuously added until the mixture of the releasing agent and the particulate material cannot form an aggregation. PA1 wherein Wr is a weight of the releasing agent used (i.e., 2 grams) and Wr is a maximum weight (g) of the added particulate material beyond which an aggregate of the mixture cannot be formed. PA1 dimethyldichlorosilane, trimethylchlorosilane, methyltrichlorosilane, allyldimethyldichlorosilane, allylphenyldichlorosilane, benzyldimethylchlorosilane, bromomethyldimethylchlorosilane, .alpha.-chloroethyltrichlorosilane, p-chloroethyltrichlorosilane, chloromethyldimethylchlorosilane, chloromethyltrichlorosilane, p-chlorophenyltrichlorosilane, 3-chloropropyltrichlorosilane, 3-chloropropyltrimethoxysilane, vinyltriethoxysilane, vinylmethoxysilane, vinyl-tris(.beta.-methoxyethoxy)silane, .gamma.-methacryloxypropyltrimethoxysilane, vinyltriacetoxysilane, divinyldichlorosilane, dimethylvinylchlorosilane, octyl-trichlorosilane, decyl-trichlorosilane, nonyl-trichlorosilane, (4-t-propylphenyl)-trichlorosilane, (4-t-butylphenyl)-trichlorosilane, dipentyl-dichlorosilane, dihexyl-dichlorosilane, dioctyl-dichlorosilane, dinonyl-dichlorosilane, didecyl-dichlorosilane, didodecyl-dichlorosilane, dihexadecyl-dichlorosilane, (4-t-butylphenyl)-octyl-dichlorosilane, dioctyl-dichlorosilane, didecenyl-dichlorosilane, dinonenyl-dichlorosilane, di-2-ethylhexyl-dichlorosilane, di-3,3-dimethylpentyl-dichlorosilane, trihexyl-chlorosilane, trioctyl-chlorosilane, tridecyl-chlorosilane, dioctyl-methyl-chlorosilane, octyl-dimethyl-chlorosilane, (4-t-propylphenyl)-diethyl-chlorosilane, octyltrimethoxysilane, hexamethyldisilazane, hexaethyldisilazane, diethyltetramethyldisilazane, hexaphenyldisilazane, hexatolyldisilazane and the like. PA1 (1) the powders of a particulate material and a releasing agent are mixed with a mixer and the like; and PA1 (2) the mixture is heated and melted while being stirred, and then the mixture is cooled.
Therefore, the actual situation is that the ultra fine particulate material is externally added to a toner as little as possible.
In addition, Japanese Laid-Open Patent Publication No. 56-1946 discloses a toner in which a particulate silica is externally-added to the toner while a particulate silica is internally-added to (i.e., is included in) the toner to enhance the reliability of the toner. However, the toner has a problem in that the fixability deteriorates because the toner has a relatively high melt viscosity.
Because of these reasons, a need exists for a toner useful for developing electrostatic latent images, which has good transferability and durability and which can produce images having good image qualities without causing the offset problem.